To fully reap their benefits, IRSs need to be properly deployed between the base station (BS) (or access point (AP)) and its served users to decrease the product-distance path loss of the BS-IRS-user cascaded channels [1]}. Towards this end, a promising solution is to deploy the IRS closer to the user terminals, e.g., in hotpot area, cell edge, or vehicles. Following this approach, the authors in [2]} considered a single-user OFDM communication system by deploying an IRS in the vicinity of a single-antenna user, where the passive reflection of the IRS was optimized to maximize the achievable rate. In [3]}, a user-side IRS was employed to assist in the communication between a multi-antenna BS and a multi-antenna user, where the capacity of the considered multiple-input multiple-output (MIMO) system was maximized by alternately optimizing the passive reflection of the IRS and the covariance matrix of the transmit signals. Besides, the authors in [4]} employed an IRS in a high-speed vehicle to enhance the communication performance between the users on board and a remote BS, where an efficient two-stage transmission protocol was proposed to conduct channel estimation and IRS reflection optimization. In addition to the user-side deployment, IRSs can also be deployed in the vicinity of the BS to assist in its communication with users distributed at different locations. In this regard, the authors in [5]} proposed an IRS-empowered BS architecture by deploying multiple IRSs close to the BS (e.g., in the range of several to tens of meters), where a novel user-IRS association design was proposed to select a certain number of cascaded user-IRS-BS channels for estimation under a pre-determined channel training overhead, and then the reflection coefficients of all IRSs were jointly optimized to maximize the minimum achievable rate among the users based on the estimated channel state information (CSI). Furthermore, a hybrid deployment strategy was proposed in [6]} to combine the complementary advantages of the BS- and user-side IRSs, which can yield higher design flexibility and provide additional passive beamforming gain by exploiting the inter-IRS reflections. In this context, the authors in [7]} investigated a double-IRS system with one IRS deployed near the multi-antenna BS and the other IRS deployed closer to the multi-antenna user, where all the involved channels were assumed to have dominant line-of-sight (LoS) paths, and the reflection coefficients of the two IRSs were jointly optimized to maximize the capacity of the MIMO system. Subsequently, the authors in [8]} extended the double-IRS system to a more practical scenario with multiple users, where an efficient algorithm by exploiting alternating optimization was proposed for cooperative IRSs' beamforming design to maximize the minimum signal-to-interference-plus-noise ratio (SINR) among all users.
